"The code is certainly there," said Mike Shaver, Mozilla's vice president of engineering, referring to parts of the Chrome Frame source code that indicate Google could crank out a Firefox plug-in similar to what Google released last week for Microsoft's Internet Explorer (IE).

Chrome Frame, which Google launched Sept. 23 , currently only supports IE6, IE7 and IE8, and lets those Microsoft browsers utilize the Chrome browser's WebKit rendering engine, as well as its V8 JavaScript engine. Google pitched the plug-in as a way to instantly boost the speed of the notoriously slow IE and as a means for Web developers to support standards IE can't handle, including HTML 5.

Specifically, Google said it had created Chrome Frame because it decided it wasn't worth the time and trouble to make its new collaboration and communications tool, Google Wave, work with IE. Google developers claimed they had spent "countless hours" tweaking Wave for IE, but gave up in favor of producing a plug-in instead.

Shaver said that Chrome Frame, which is open source, includes technologies that could be destined for Firefox as well as Opera Software's Opera browser. "They've chosen not to ship yet," said Shaver, talking about plug-ins for those two browsers. "I hope they won't."

Mozilla staked out its position on Chrome Frame Tuesday, when both Shaver and Mitchell Baker, the former CEO of Mozilla and currently the chairman of the Mozilla Foundation, took swipes at Google for releasing the plug-in.

Both Shaver and Baker called the plug-in a bad idea, with Baker arguing that it would confuse readers over which browser was rendering a site, divide personal details like site passwords and browsing history between two applications and cede control over users' browsing experience to site designers.

Shaver, meanwhile, panned Chrome Frame for some of the same things that Microsoft used to hammer Google earlier, including breaking IE's private browsing mode.

Shaver acknowledged that some of Mozilla's motivation for taking Chrome Frame to the woodshed was in its own interest. "We started to get some inbound [feedback] from our community about Chrome Frame, and the more we did, the more we decided that we'd rather have people using Chrome the browser rather than Chrome Frame.

"And we wouldn't want Google to do for us what they did for Microsoft," Shaver said.

Google's intentions are unclear, Shaver admitted, and Mozilla has not talked directly with Google about Chrome Frame. For its part, Google did not reply to queries made early Wednesday asking whether it plans to craft a plug-in for browsers other than IE and why.

Firefox, although not as fast as Google's Chrome browser in rendering JavaScript, is no IE: In recent tests by ComputerworldChrome raced through the SunSpider benchmarks almost 10 times faster than Microsoft's IE8, but only about two-and-a-half times faster than Firefox 3.5.

Shaver also had some technical issues with Chrome Frame, or rather Google's concept. "We don't need to repeat the problems with Flash," said Shaver. "Developers program to the limitations and capabilities of a particular version of Flash, which then makes it difficult or impossible for others [to use a site]. And we know how hard it is to get users to update their plug-ins."

Earlier this month Mozilla piggybacked a new check for outdated versions of Flash with its latest Firefox security update in an attempt to get users to download and install the newest, and thus the most secure, version of the popular Adobe plug-in.

If Chrome Frame becomes widely used, Shaver said, site and Web application developers might program specifically for Chrome Frame's HTML 5 support, to the detriment of the rest of the Web's users, who overwhelming use rival browsers like IE, Firefox, Apple's Safari and Opera.

Although HTML 5 has been proposed as a standard, it has not been officially adopted, and many parts of the specification are up for grabs.

"The behavior of [a user's] browser is seriously hindered by delegating the choice of software to the developers of individual sites they visit," said Shaver in a blog post he wrote Tuesday. "It is a problem that we have seen repeatedly with other stack plug-ins ... and not one that I think we need to see replayed again under the banner of HTML 5."

The head of Microsoft's IE group, Amy Bazdukas, had the same thoughts earlier this week when she knocked Google for coming up with Chrome Frame. "HTML 5 is not a completed standard," Bazdukas said in an interview Monday. "We're working on it very actively, and we see a lot of promise in it. But it's premature to support it."

Shaver was unsure how Mozilla would react to a Chrome Frame plug-in for Firefox, or whether it even would. "I don't know, although we are doing a bunch of work on isolating the plug-in," he said.

"But I'd be unpleasantly surprised if Google did do a [Firefox] plug-in," Shaver concluded.